The overall goal of this revised proposal is to determine the biologic effects of a novel breast tumor antigen containing F-box and WD40 domains, with an emphasis on its roles in age- and T cell-dependent susceptibility to tumor growth. We have identified a novel human breast tumor antigen (HBTA1) that strongly inhibits proliferation of T cells from aged, but not young, mice. Immunization of 2-mo-old BXD 12 mice with HBTA1 protects against the tumor challenge at 16-mo-of-age. Sucrose gradient purification of a mouse homologue HBTA1 (mHBTA1) followed by electromicroscope examination of the purified samples led to the realization that mHBTA1 is packed in the exosomes of the TS/A tumors. The TS/A exosomal mHBTA1 strongly inhibits the proliferation of T cells isolated from aged, but not young, BXD12 mice. The ability of T cells to proliferate in the presence of TS/A exosomes is correlated with the ubiquitination status of exosomal mHBTA1. Our aims are (1) To determine if ubiquitination of mHBTA1 is correlated with susceptibility to tumor growth and the tumor-specific cytotoxic T-cell response. Aged-mice will be immunized with ubiquitinated exosomal mHBTA1 to determine its effects on enhancement of protection against the tumor growth. In parallel, we will determine if the production of endogenous ubiquitinated mHBTA1 in TS/A exosomes is correlated with the tumor susceptibility. (2) To use siRNA technology to determine if knockout of exosomal mHBTA1 of tumor cells is sufficient to reverse the inhibition of T cell proliferation mediated by TS/A exosomes or whether other exosomal proteins are required. (3) To determine if the age-dependent T cell factors we recently identified play a role in ubiquitination of exosomal mHBTA1, and thus inactivation of mHBTA1-mediated inhibition of T-cell activation;we will further determine the pathway by which TS/A exosomal mHBTA1 interacts with T-cell factors. (4) To determine if an age-related T-cell factor(s) regulates the ubiquitination of exosomal HBTA1 of human breast tumors, and, further to determine its possible association with the age-related incidence of breast cancers. The data generated should elucidate the cellular and molecular basis for the role of tumor exosomes and exosomal mHBTA1 in the development of age- and T cell-dependent breast cancer.